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Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Of all the different types of reactions molecules participate in, probably 90% of them involve polar reaction mechanisms. By polar reaction mechanisms what I'm describing is that the bonds that are being made or broken occur heterolytically. So typically two electrons that are in a bond will be broken such that one of the two atoms gets both of the electrons.
Now the first type of reactivity that we're going to focus on is the type involving hydrogen, and in particular, hydrogen connected to a relatively electron deficient atom, an electro negative atom, such that this bond is polarized towards that electron deficient atom.
Now what I want you to see is that the greater that bond is polarized, the bigger the difference in electronegativity between these two atoms, the easier it will be for a base to come and pick up the proton without any electrons leaving the pair of electrons that were in this bond behind with A.
Now to demonstrate this, we want to do a highly technical demonstration. I need to call in my colleague here. Gordon, if you would please? Let's suppose that this red baton represents two electrons for chemical bond. If we both have equal hold, in other words, what this represents is equal electronegativity between the two atoms, pink atom and green atom, then it's going to be relatively difficult for either one of us to get away with both of those electrons. On the other hand, if Gordon has a weaker hold on the electrons, it will be very easy for me to get away with both of the electrons. Again, a heterolytic cleavage and so if Gordon is a proton, it's going to be easier for a base to get him away with something else leaving the pair of electrons with me, the "A" atom in this particular case. So once again, what's crucial for you to understand here is the more that this bond is polarized, the easier it will be towards "A," the easier it will be to remove the H^+ leaving the electrons behind.
So that's going to be our tool for predicting acidity of molecules. You've looked at lots and lots of examples of acids, but what we haven't done is talked about what makes them acidic. And this is the basic idea behind it. So the first thing we're going to look at is the bond strength, the character of the bond that is between the hydrogen and the atom it's connected next to. Eventually we'll look at other effects, but at first we're just going to look at that bond.
Now the first trend that I'd like to talk about is working across the periodic table, going from carbon to nitrogen to oxygen to fluorine. As we go across this period of the periodic table, what is happening? Well, the big difference, carbon, nitrogen, oxygen and fluorine is that the electronegativity is increasing. Remember, electronegativity increases going from left to right in the periodic table. When that occurs the bond between the hydrogen and that atom becomes more polarized. The more it is polarized, the easier it is to remove as H^+. Therefore, the stronger acid it is. How do we measure acidity? K[a], right? The acid dissociation constant or pK[a], the power of the acid dissociation constant.
So here are the pK[a] values and what I want to show you in particular is the incredible difference in pK[a ]just between these two even, HF and H[2]O. Here we have a relatively strong acid. Now to clarify, this is still a weak acid by our formal definition in that it does not completely dissociate in water, but none the less it is much stronger than water itself is a good eleven orders of magnitude difference between the two. Now remember, this is not 11 times, this is 11 powers of ten difference between the acidity of these two species - a huge difference. Well, just look at how pathetic methane is as an acid. This means it's K[a] is 10^-49. If I had a solution of methane, I would have less than one molecule per mole, 10^23 of methane that had actually lost a proton. Essentially it just doesn't lose a proton. This thing is such a weak acid. Ammonia, again, is somewhere in between these. So you see huge, huge differences in acidity simply by going from carbon to nitrogen to oxygen to fluorine. And that again, reflects the bond between hydrogen and whatever it's connected to.
Now that's all fine and good. But let's talk about a different trend. Let's look down the periodic table instead of across the periodic table. What we'll do as an example is we'll look at the halides, HF, HCL, HBR and HI and let me ask you again, what's happening in terms of electronegativity. Well, if you remember the trend as we go down the periodic table, the electronegativity decreases meaning of all of these, fluorine is the most electronegative and we should expect the bond to be most polarized and therefore, easiest to break heterolytically and we'd expect HF to be the strongest acid. And that's exactly what we don't see. The trend, in fact, is exactly the reverse of that. HI is the strongest acid. Remember this is pK[a] so the negative number means super powerful acid and then still very, very strong and then only HF is considered weak by our standards. So strongest acid to weakest acid, HI being the strongest acid now rather than what we expected.
Okay, red flags are going up all over. This is telling us that there is something fundamentally different here, a different trend at work than the last trend we saw. This clearly can't be explained by electronegativity, but what it does have everything to do with is the actual bond strength. Now this could be confusing because weren't we just talking about bond strength? How polarized the bond is is one way of describing that bond, but how strong it is, what's the energy stabilization in making the bond, that's a whole different idea. And, in fact, the overlap between the atomic orbitals of hydrogen and iodine is very poor. And so we have a very weak bond as a result of that. While HF has an extremely strong bond, it costs us 563 kJ to break that bond, almost half of that to break the HI bond.
So let me elaborate on this a little bit further. Let's talk about going from HX to H^+ and H^-. This is the overall reaction that we're talking about, losing a proton, right? This is the heterolytic cleavage that we're talking about. But I could imagine that happening as a series of steps just in terms of energy. This is a thought experiment. I could imagine breaking the bond homolytically into two individual atoms, one electron to each, and that is what these numbers measure. That's the bond dissociation energy. And then I could imagine taking an electron away from hydrogen, adding an electron to whatever X is and that, again, would be electron affinity if I do that. Don't forget, taking away the electron is ionization energy. This is a constant. That's going to be the same because it's hydrogen in all of these cases so I won't pay any attention to this one. But what I'm saying is that the energy involved in the heterolytic cleavage, to go from here to here, is simply the sum of the energies to break it homolytically, the energy it costs us to pull that electron out and the energy we get back by giving an electron to this atom to make X^-. Now when we go across the periodic table, this is the term that is governing what's going on. The electron affinity changes quite a lot going from carbon to nitrogen to oxygen to fluorine, so that's the dominant term that's dictating how easy it is to split it. In other words, the more polarized the bond is, the easier it is to break it. Just like we said before, but when we go down the periodic table, this term doesn't change all that much. It changes some, but not enormously. But what does change to an enormous amount is the bond dissociation energy. It's a much weaker bond break for HI than for HF and so that becomes the dominant term when we're talking about going down the periodic table and comparing acidities. So simply because the bond is a lot weaker for HI than HF, the trend is strongest acid to weakest acid, completely opposite from what we predict based on electronegativity.
So the two trends to remember, and again, these are broad generalizations, but for good reasons. As we go across the periodic table from left to right, acidity will increase and as we go down the periodic table from top to bottom, acidity is going to also increase. Now I have to remind you of one thing. All of this pertains only to the situation where hydrogen is directly connected to some other element that we're talking about. If there are other elements that are changing further away, that's a whole different matter and we'll look at that next. But what we're worried about right now is the nature of the bond itself that's being broken; hydrogen connected to iodine, hydrogen connected to fluorine, hydrogen connected to oxygen and so forth. The two trends that I just mentioned strictly pertain to those situations. When the variable that we're changing is the atom that hydrogen is connected with.
Well, again, this is only one of several effects that determine acidity so let's go ahead now and take a look at inductive effects and how they effect the acidity of the molecule.
Introduction to Organic Reactions
Acid Strength in Organic Molecules
Inductive Effects Page [1 of 2]

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